Microwave rotary joint



April 8,'1958 J. F. zALl-:sKl

MICROWAVE ROTARY JOINT Filed June 25, `1954 9000 9500 ESRCYCL ES PE2 5CON0 INVENTOR.

United States Patent ias Y z,sso,z76 MICROWAVE noTAnY sonar John F. Zaleski, Valhalla, N. Y., assigner to General Precision Laboratory Incorporated, a corporation of New York Application June 25, 1954, Serial No. 439,228

Claims. (Cl. S33- 98) This invention relates to a rotary joint for connecting two rectangular hollow microwave guides and more particularly to such a rotary joint employing two rectangular to coaxial waveguide transitions.

One method of constructing a rotary joint between two rectangular guides consists of inserting a length of coaxial line between the two guides and perpendicular to both of them. A rotating joint is provided within the coaxial line, such a joint not disturbing the radial TEM mode of transmssion of energy therein. However, in such a device great difficulty has been experienced in effecting the etlcient transfer of energy between the coaxial line and the rectangular guides, and particularly in effecting such transfer over a wide frequency band, with constant impedance over the band and with no .detectable propagation of higher order modes.

That is, a broadband rectangular to coaxial transition preferably has a low voltage standing wave ratio (VSWR) over a continuous wide band of frequencies. A broadband rectangular to coaxial rotary joint in addition must not readily propagate higher order modes in order to keep its coaxial length to a minimum. Available rectangular to coaxial transitions have been only relatively satisfactory in these respects. It is rare indeed to nd any transition providing a rotary joint bandwidth wider than 10% between 1.1 voltage standing wave ratio (VSWR) points. In addition when two such transitions are cornbined in a rotary joint the unavoidable generation by each of higher modes has required that they be connected through a relatively long coaxial line to permit these higher modes generated at one transition to die out before they reach the other transition.

The present invention provi-des an ecient broadband rotary joint employing two rectangular vto coaxial transitions. porates a novel principle differing from that used in one form or another in all previous transitions. Instead of coupling each rectangular guide to the coaxial line in a manner to produce geometrical and impedance discontinuities, then providing compensating ydiscontinuities or tuned elements, as is done in prior art, the present invention introduces a novel short-circuiting and coupling strap through which all but a modicum of the discontinuous effect of the transition is eliminated, and which obviates the need for tuning elements or matching-out components.

The rotary joint of the present invention has an operating bandwidth approximately twice as wide as that of the best previously available rotary joints, with energy dissipation negligibly low throughout this wide band. Additionally, the. instant joint is very simple in design, has but two critical dimensions in each of its transitions, and permits wide manufacturing tolerances even in these diinensions` In sum, the instant joint is uniquely superior in widebanding, in eiliciency and in compactness. Thus the unique principles underlying this rotary joint result in properties, so much better tha-n previous. joints that the instant invention marks a qualitative advance in the art.

The present rotary joint contains two usually identical These transitions are of a design which incor- I "ice transitions, each of which has as its novel feature a metal strap mechanically supporting the inner coaxial conductor from the outer coaxial conductor at the plane of the rectangular guide face, and electrically connecting and short-circuiting the two coaxial conductors in the median line of the rectangular guide broad face. Although the metal short-circuiting strap might be thought to short-circuit all applied microwave energy and permit none to be transmitted, and although such a short-circuit design element certainly cannot be arrived at a priori by any extension of existing transition theory, nevertheless the physical fact of operability has been proved, and moreover, of superior operability. Additionally, the rectangular guide is carried beyond the transition for a short distance and terminated in a short-circuiting plate, and the inner coaxial conductor is extended to form a probe within the rectangular guide.

The principal object of the invention is to provide a microwave rotary joint having a broadband transmission characteristic. y

Another object is to provide a rectangular-coaxial transition with negligible absorbed power loss over a very broad band of frequencies.

Another object is to provide a simply designed eiiicient and broadband rotary joint having the fewest possible parts which at the same time is readily reproducible.

Another object is to provide a simply designed ecient and broadband rectangular-coaxial transition which is readilyreproducible and which has the fewest possible arts. p Further understanding of this invention may be secured from the detailed description together with the associated drawings, in which;

Figure l is an isometric view of a microwave rectangular coaxial joint embodying the invention.

Figure 2 is a side View of the rotary joint of Fig. l sectioned at the coaxial axis.

Figure 3 depicts a choke joint for alternative use in the rotary joint of Figs. 1, 2 and 4.

Figure 4 is an end view of the rotary joint of Fig. 2 sectioned on line 4 4.

Figure 5 is an elevational cross section of a rectangularcoaxial junction embodying the invention, having the strap recessed at the rear side of the coaxial line.

Figure 6 is an elevational cross section of a rectangularcoaxial junction embodying the invention employing a coupling septum to support the inner coaxial conductor.

Figure 7 is a graph depicting the wide band characteristic of the rotary joints of Figs. l, 2, 3 and 4.

The following detailed description of a rotary joint embodying the invention covers those components depicted in Fig. l, which are very generally and briey described as constituting a rotary joint between two rectangular guides 11 and 12 having relative motion in parallel planes. These guides 11 and 12 are orthogonally connected by a short length of coaxial line 13 with axis intersecting theV guides axes. The coaxial line 13 containsL a rotatable juncture 14. The coaxial line 13 at its junction with each rectangular guide includes provisions for the eilcient transfer of microwave energy between the two kinds of transmission members, these provisions being termed, at each end of the coaxial line, a rectangular waveguide to coaxial line transition. These transitions are generally of the same design but need not be and can embodydifrerent variations of this invention. In every case, however, each transition consists of three principal parts which are, as identiied in the upper of the two transitions shown in Fig. 1, a probe 16, a strap 17, and a short-circuited stub consisting of the -continuation of the guide 12 beyond the transition to form a very short guide. length 18, terminated by atran'sverse metal shortcircuiting plate 19.

Patented Apr. 8, 1958 y tion.

upon the`guide dimensions which have been chosen. Al-' though conventionally the two rectangular guides 11 and 12 are of the same size, they need not necessarily be so,

in which case the two transitions would have Vsomewhat l different dimensions. v. j

The transition dimensions are also dependent to some extent upon the size of the coaxial line. This size is chosen for efficient propagation' of energy of the desired frequency band in the'TEM mode. Its arithmetic mean diameter must be held below a selected value, as is well A understood in the art, to prevent effective propagation of higher modes. In addition, the coaxial line must be long enough to permit what higher modes are unavoidablj generated at a transition to die out before reaching Vthe other transition. Additionally it is desirable that the diameters of the inner and outer conductors be so selected as toA afford the maximum airgap therebetween so that the greatest amount of power may be transmitted without danger `of arc-over. The rotatable juncture 14 of the coaxial line is most simply made with mortised butt joints, which are fairly efficient if accurately made. Such joints are employed in Fig. 2, illustrating the juncture 14 of Fig. .1 in cross section. The upper part 21 of the outer conductor is mortised into the lower part 22 with a running tit, and the upper part 23 of the rod composing the inner conductor is terminated in an axial rod fitting in an axial hole in the lower part 24. The joints ofthe outer and inner conductors should be displacedy axially by one-quarter, wavelength to neutralize their discontinuity etects.

Fig. 3 illustrates a rotatable juncture of the choke type which may be used'in place of the mortised butt point of Fig. 2 if desired. v

The total length of the annular choke space 26 in the outer conductor is made equal to one-half wavelength to produce low impedance at the joint line 27 in the inner surface of the outer conductor. Similarly, the total length of theannular choke space 28 in the inner conductor is made equal to one-half wavelength to provide a low impedance joint 29. The outer joint 31 is onequarter wavelength from the closed end 30 of the annular space 26, which places the joint 31 at a high imped-V ance point in the half wavelength line effectively `minimizing any radio frequency losses or leakage at this junc- The straps employed in the rectangular-coaxial transitions are exampled by strap 17, Fig. l. This strap 17 is a metallic strip or bar conductively secured at one end by soldering, brazing, riveting or other means to the innerface 32 of rectangular guide 12 with the median longitudinal line of strap 17 parallel to the median longitudinal line [of face 32. The other end of strap 17 projects radially into the coaxial line 13 parallel to the plane of the guide wall 32 and is secured to the inner coaxial conductor 23, supporting it mechanically and of course apparently short-circuiting it electrically to the outer conductor. Actually the electrical functions of the strap include assisting in coupling the rectangular guide energy into the coaxial line or vice versa. The end of the strap which is secured to the inner coaxial conductor is rounded to the radius of the coaxial inner conductor.

The strap 17 is provided at its end with a hole concentric with the coaxial line. The coaxial inner conductor 23 s extended in a thin pin' 16 which passes through the hole and is a pressed fit therein. The pin 16 thus extends beyond the strap and the inner face of guide wall 32 to constitute a probe or antenna extending in the position of the maximum electric'eld and parallel thereto within the `rectangular guide. The pin 16 is therefore in the position and attitude for eiective electric coupling tothe guide field, the degree of coupling being dependent upon the probe length. The probe length is very roughly one-third of the lesser cross-sectional guide dimension. Since the probe penetration required is much less than ordinarily required in transitions between rectangular waveguide and coaxial lines the possibility of arc-over between the probe end and the waveguide wall is greatly minimized. The probe 16 preferably has a rounded end to fur-ther decrease possibility of arc-over fromA this point to the opposite waveguide wall.

The guide 12 is extended with its cross-sectional dimensions unchanged beyond the transition to form a stub 18 short-circuited by the metallic plate 19, which is positisned perpendicularly to the guide axis. The length of the stub measured from the coaxial line axis is on the order of the radius of the coaxial line. That is, the surface of the short-circuiting plug is very roughly and approximately at the position of the Vouter conductor of the coaxial line. The short-circuiting plug has a ilat surface within the guide perpendicular to the guide and completely blocking it, as indicated in Figs. 1, 5 and 6.

The surface of the short-circuiting plug, however, may be provided with other configurations if so desired. For example, as disclosed in Figs. 2 and 4, the shorting plugn19 may be provided with a cylindrical recess 20. This construction is used where a particularly desired ycenter impedance matching frequency necessitates moving the shorting plug 19 in the direction of the coaxial line to a location such that the shorting plug would tend to partially cover the coaxial line aperture. Cylindrically recessing the end plug will eliminate the possibility of arc-over in this region.

Asany alternative construction the strap 17 as depicted in Figs. l, 2 and 4 may extend in the opposite direction as indicated in Fig. 5. In this construction the strap 34 extends from the inner conductor of the coaxial line `towards and along the rectangular shorted stub 35. Ad-

ditionally as also depicted in the modification of Fig. 5, the strap 34 may be recessed or inserted in the broad face of the rectangular waveguide. Such recessing which may be accomplished whether the strap is oriented in the direction towards the waveguide stub as in Fig. 5 or in the direction of the waveguide sections 11 and 12 as in Figs.

l, 2 and 4'has the advantage of eliminating the` discontinuity causedvby the exposed end of the strap. In such constructions the efficiency of the transition and hence the rotary joint is slightly increased although the construction is slightly more complex.

A more rigid construction, where such is desired, is illustrated in Fig. 6. In this form of the invention the probe and strap are combined in a single septum-like element which performs the functions of both elements. In such an arrangement the width of the septum is made equal to the probe length and has a thickness equal to the diameter of the inner coaxial conductor with its forward edge rounded to equal the radius of curvature of: the coaxial inner conductor.

The general construction of the rotary joint of the invention and the transitions used therein as heretofore ydescribed sets forth the inventive concept and a device constructed according to such description will exhibit characteristics, particularly with respect to being broadband, superior to what has heretofore been obtainable.

Of course, one skilled in the art will recognize that in any particular application and for any design frequency the various dimensions of the structure must be so selected as to yield the best results. Y

As an aid in arriving at the proper selection of dimensions and by way of example rather than limitation the following examples may be set forth.

In a construction such as shown in Figs. 2 and 4 designed to be centered at 9000 m. c. p. s. standard rectangular waveguidev having internal dimensions of 0.4 x 0.9 and coaxial guide, the-internal conductor diameter of which is 0.1875 inch and the internal diameter of the outer conductor of which vis 0.531 inch, may be used. The straps 17 have a width equal to the diameter of the coaxial inner conductor and are made to have a thickness of 0.05 inch. The probes 16 Aand 16 have a diameter of 0.125 inch and extend into the rectangular waveguide sections by an amount such that the distance H from the probe tip to the opposite wall is 0.261 inch, this indirect method of measurement being the most convenient to utilize in constructing such a device. The terminating short-circuiting blocks 19 and 19' are provided with concave recesses 20 having a radius of 0.316 inch extending into the blocks to Va depthof .055 inch. Likewise the blocks are so positioned that the distance between the axis of the coaxial waveguide and the plane of the flat surfaces is 0.2615 inch. S-uch a joint exhibits the characteristic illustrated in Fig. 7 having a bandwidth between the usually accepted points of 1.10 VSWR of from 8000 to 1000 m. c. p. s., or over 22%. Additionally the VSWR between 8120 and 99.20 m. c. p. s. is below 1.06 which indicates that manufacturing tolerances are not critical and that excellent performance can be -obtained even though the d-imensions are varied somewhat from those set forth. A VSWR of 1.1 of course, corresponds to less than 0.23% power reflection while a VSWR of 1.06 corresponds to less than 0.09% power reflection indicating that reflected power loss is negligible over an extremely wideband nearly twice that obtained in any heretofore known design.

In addition, absorbed power losses (12R losses in the conducting surfaces) due kto the transitions are less than 0.1 db for the lrotary joint.

As an example of how the critical dimensions, namely, the distance between the probe tip and the opposite wall and the distance between the short-circuiting plug and the axis of the coaxial guide may be varied, the plug recess may be reduced in depth to .045 inch the flat surface positioned 0.263 inch from the axis of the coaxial guide while retaining such insertion of the probe as to provide a clearance distance H of 0.261 inch and the bandwidth between 1.1 VSWR points will still amount to 20%. n

Using the same recess in the short-circuit plug but adjusting it so that the flat Isurface is positioned at a distance of 0.271 inch from the axis of the coaxial guide and adjusting the probe to a clearance distance H of 0.262 inch a device having a 22% 'bandwidth is obtained.

When a flat faced lshort-circuiting plug is used the r plug distance and probe depth should be varied som-ewhat, for example, in a construction of this type if the distance between the flat face of the short-circuit plug and the axis of the coaxial guide is made 0.215 inch and the probe is adjusted to a clearance distance H of from 0.234 to 0.238 inch a 19% bandwidth between 1.1 VSWR points is obtained.

If the strap 17 is recessed in the face of the waveguide so that it is flush therewith some change in the probe ydepth and plug distance is required. For example, using a construction as disclosed in Figs. 2 and 4 except for at faced plugs and recessed straps a bandwidth of 18% between 1.1 VSWR points is obtained when the distance between the face of the plugs and the axis of the coaxial guide is 0.312 inch and the probe clearance distance H is 0.265 inch.

Additionally orienting a recessed strap in the manner depicted in Fig. 5 requires some change in dimensions. In such an arrangement using a flat faced plug positioned at a distance of 0.378 inch from the axisV of the coaxial guide with a probe clearanceof 0.220 inch the excellent bandwidth characteristic of 26%` between 1.1 VSWR points was obtained while 18% bandwidths were obtained when the plug distance was 0.370 inch and the probe clearance 0.235 inch on the one hand and when the plug distance was 0.461 inch and the probe clearance 0.225 inch on the other.

`The numerical examples here given of course do not preclude the use of other relative adjustments but merely point the way to achieving the best results. In any event, since the'number of critical dimensions are few in nurnber the proper design for any particular application and frequency is relatively easily obtained. t v

What is claimed is:

l. A transition comprising, a rectangular waveguide section, a coaxial line having its outer conductor orthogonally joined to a broad face of said waveguide section intermediate the ends thereof, means short circuiting one end of said waveguide section adjacent said coaxial line, and elongated conductive member electrically interconnecting the internal conductor of said coaxial line with the outer conductor thereof and the broad face of said waveguide section, said elongated conductive member being positioned in said waveguide section and having a surface :thereof lying inA the -plane of said broad face and extending longitudinally thereof, and a conductive extension of said inner conductor extending into said waveguide section beyond said surface a distance less than the height of said waveguide section.

2. A transition comprising a rectangular waveguide section, a coaxial line having its outer conductor orthogonally joined to abroad face of said waveguide section midway between the narrow sides thereof, said waveguide section being prolonged beyond the axis of said coaxial line to form a stub, a metallic plug transversely short-circuiting said stub, a conductive strap electrically interconnecting the inner conductor of said coaxial line withthe outer conductor thereof and the broad face of said waveguide section, said conductive strap being positioned in said waveguide section and having a face thereof lying in the plane of said broad face and extending longitudinally thereof, and a probe prolongation of said inner conductor extending into said waveguide section beyond said strap a distance less than the height of said waveguide section.

3. A transition comprising a rectangular waveguide section, a coaxial line having its outer conductor orthogonally joined to 4a broad face of said waveguide section midway between the narrow sides thereof, said waveguide section being prolonged beyond the axis of said coaxial line to form a stub, a metallic plug transversely short-circuiting said stub, a conductive strap electrically interconnecting the inner conductor of said coaxial line with the outer conductor thereof and the broad face of said waveguide section, said conductive strap beingy positioned in said waveguide section and having a face thereof lying in the plane of said broad face and extending longitudinally thereof in a direction away from said stub, and a probe prolongation of said inner conductor extending into said waveguide section beyond said strap a distance less than the height of said waveguide section.

4. A transition comprising a rectangular waveguide section, a coaxial line having its outer conductor orthogonally joined to a broad face of said waveguide section midway between the narrow sides thereof, said waveguide section being prolonged beyond the axis of said coaxial line to form a stub, a metallic plug short-circuiting said stub, a conductive strap electrically interconnecting the inner conductor of said coaxial line with the outer conductor thereof and the broad face of said waveguide section, said conductive strap being positioned in said waveguide section and having a face thereof lying in the plane of said broad face and extending longitudinally thereof in a direction towards said stub, and a probe prolongation of said inner conductor extending into said waveguide section beyond said strap a distance less than the height of said waveguide section.

5. A transition comprising a rectangular waveguide section, a coaxial line having its outer conductor orthogonally joined to a broad face of said waveguide section midway between the narrow sides thereof, said waveguide section being prolonged beyond the axis of said coaxial line to form a stub, a'rnetallic plug short-circuiting said stub, a conductive septum electrically interconnecting the inner` conductor of lsaid coaxial line with the outer conductor thereof and the broad face of said waveguide section said septum extending in a plane defined by the axis of said coaxial line andthe longitudinal median line of said waveguide section, said septum-lying wholly in said waveguide section and having oneedge thereof lying in the plane of said broad face and the other edge thereof spaced from the internal surface of the opposit broad face ofsaid waveguide section.. y Y

6. A rotary joint comprising, a pair `of rectangular waveguide sections, a coaxial line containing a rotatable juncture having its outer conductor connected to and joining broad faces of said pair of waveguide sections, each of said waveguide sections being provided with a short-circuited prolongation extending beyond said coaxial line, a pair of elongated' conductive members each electrically interconnecting` the internal conductor of said coaxial line with the outer conductorthereof and the broad face` of a `respective waveguide section, said elongated conductive members each being positioned in a respective waveguide section and each having a surface thereof lying in the plane of the respective broad face to which it is connected and `extending longitudinally thereof, said inner conductor being provided with conductive extensions at each end extending into a respective waveguide section beyond the surface of `a respective elongated conductive member a distance less than the height of a respective waveguide section.

7.` A rotary joint comprising, a pair of rectangular waveguide sections, a coaxial line containing a rotatable juncture having its outer conductor connected to and joining broad faces of said pair of waveguide sections, each of said waveguide sections being provided withv a short-circuited prolongation extending beyond said coaxial line, a pair of conductive straps each electrically interconnecting the internal conductor of said coaxial line with the outer conductor thereof and the broad face of a respective waveguide section, said conductive straps each being positioned in a respective waveguide section and each having a face thereof lying in the plane` of the respective broad face to which it is connected and extending longitudinally thereof, said inner conductor being provided with probe extensions at each end extending into a respective waveguide section beyond ,a

respective strap a distance less than the height of a respective waveguide section.

y 8. A rotary joint comprising, a pair of rectangular waveguide sections, a coaxial line containing a rotatable juncture having its outer Aconductor connected to and joining broad faces of said pair of waveguide sections, each` of said waveguide sections being provided with a shortcircuited prolongation extending beyond said coaxial line,

a pair of conductive straps each electrically interconnecting the internal conductor of said coaxial line with the outer conductor thereof and the broad face of a respective waveguide section, said conductive straps eachbeing positioned in a respective waveguide section and each having a face thereoflying in the planeof the respective broad face to which it is connectedvand extending longitudinally thereof in a direction away from said short-circuited prolongashort-circuited prolongation extending beyond said coaxial line, a pair of conductive straps each electrically interconnecting the internal conductor of said coaxial line with the outer conductor thereof and the broad face of a respective waveguide section, said conductive straps each being positioned in a respective waveguide section and each having a face thereof lying in the plane of the respective br-oad face to which it is connected and extending longitudinally thereof in a direction towards said short-circuited prolongation, said inner conductor being provided with probe extensions at each end thereof extending into a respective waveguide section beyond a respective strap a distance vless than the height of a respective waveguide section.

10. A rotary joint comprising, a pair of rectangular .waveguide sections, a coaxial line containing a rotatable juncture having its outer conductor connected to and joining broad faces of saidpair of waveguide sections, each of said waveguide sections being provided with a short-circuited prolongation extending beyond said coaxial line, a pair of conductive septums each electrically interconnecting the` inner conductor of said coaxial line with the outer conductor thereof and the broad face of a respective waveguide section, each of said septums extending in a plane defined by the axis of said coaxial line and the longitudinal median line of the respective waveguide section to which it is connected, each of said septums lying wholly in a respective waveguide section and each having one edge thereof lying in the plane of a broad face of a respective waveguide section with the respective opposite edges thereof spaced from the internal surface of the respective opposite broad faces of the' waveguide section in which the respective septum is located.` i

Zaleski Dec. 23, 1947 Fano Nov. 25, 1952 OTHER REFERENCES Ragan, Microwave Transmission Circuits, vol.` 9, M. I. T. Radiation Laboratory Series, published 1948 by McGraw-Hill, pages 407-16 and446 relied on. 

